A number of industrial applications require a suitable vacuum to be kept in a sealed space for a period of several years. In particular, electronic vacuum devices such as CRTs (Cathode Ray Tubes), which are used as screens of television sets or computers, have this requirement. In CRTs, vacuum is required to avoid electrons emitted by a cathode from being deflected by collisions with gas particles. CRTs are evacuated during the manufacturing step through mechanical pumps and then hermetically sealed.
The vacuum in the tube tends however to decrease during time, mainly because of the degassing from internal components of the tube. It is therefore necessary to use inside the tube a getter material capable of capturing the gaseous molecules, thus preserving the vacuum degree necessary for the cathodic tube to work for the time needed. For this purpose barium is usually used in the form of a thin film deposited on inner walls of the cathodic tube. Because of the high reactivity of this metal, which would make every manufacturing operation troublesome, barium is used in the form of the air stable compound BaAl4. To introduce the compound inside the cathodic tube there are utilized the so-called “evaporable getter” devices, formed of an open metallic container, inside which there is a compressed mixture of BaAl4 and nickel powders (in a weight ratio of about 1:1); devices of this type are disclosed for example in patents U.S. Pat. Nos. 2,842,640, 2,907,451, 3,033,354, 3,225,911, 3,381,805, 3,719,433, 4,134,041, 4,486,686, 4,504,765, 4,642,516 and 4,961,040. These patents are incorporated herein by reference, in particular, for their teaching of methods of vaporizing BaAl4 alloys within a sealed vacuum chamber, and various electronic devices employing such getters.
The BaAl4 alloys are introduced inside the cathodic tube before sealing it, and then are heated from outside through radio frequencies to cause the evaporation of barium, which then condenses on the internal walls thus forming the film active in sorbing gases. Nickel has the function of reducing the energy required at radio-frequency heating: when the temperature of the mixture reaches about 850° C., the following exothermal reaction takes place: BaAl4+4 Ni→Ba+4 NiAl. The heat generated by this reaction raises the temperature of the system up to about 1200° C., necessary to have barium evaporation; these devices are defined “exothermal” in the field.
The use of barium, however, has some drawbacks. First of all, like all heavy metals, it is a toxic material, so that the more barium material used, the more precautions that must be taken in manufacture, and also the greater the problems associated with disposing of the device to avoid environmental contamination. Furthermore, inside the cathodic tubes, barium is present also in areas hit by highly energetic electron beams used to generate the image inside the kinescope; in these conditions barium, and consequently the screen of the kinescope, emit X rays (even though in small quantities) that may be harmful to health.
In order to avoid the problems caused by the use of barium, co-owned PCT application WO 01/01436, discloses the use of calcium as a gas sorbing getter material, and the compound CaAl2 as a precursor to be utilized for evaporating calcium. The compound CaAl2 is preferably used in mixture with titanium powders.
The use of calcium-based evaporable getter material has also some advantages during the manufacture of CRTs, in that the evaporation of calcium is less violent and more easily controllable with respect to barium, even after the treatments at relatively high temperatures (about 450° C.) in oxidizing atmospheres which occur during some of the manufacturing steps of the tubes.
However, the calcium getter material disclosed in the above WO 01/01436 application has the problem that the CaAl2 alloy accumulates a substantial amount of hydrogen during its manufacture. The hydrogen contained in the alloy is released during the evaporation of calcium, and can negatively interfere with the deposition process. Furthermore, it is known that hydrogen can react with carbon atoms on the surface of metallic films, forming low molecular weight alkanes, such as methane, which is reabsorbed only with difficulty and partially by the same film.